nextjs-app-router

Next.js 16 is dynamic by default, so stale data means something explicitly opted into caching. Check for fetch calls with next.revalidate, or a 'use cache' directive on the route or a called function, then invalidate with revalidateTag or revalidatePath after mutations.

Place 'use server' at the top of a file and validate all input with Zod safeParse before touching the database, since clients can send anything. Return an error object for handled failures and call revalidatePath or revalidateTag after the mutation.

revalidatePath re-renders an entire route segment, while revalidateTag invalidates specific tagged cache entries across multiple routes. Use tags when the same data appears on more than one route, since they are cheaper and more targeted.

No, middleware.ts was renamed to proxy.ts in Next.js 16, including the exported function name, though the logic is identical. Use it for bulk route protection at the edge, and use layout-level auth() checks for per-route role logic.

A 'use client' layout that gates children behind client-only state never renders the subtree server-side, so raw SSR HTML contains only the fallback. Make the layout a server component and resolve gating data from cookies or headers in the RSC.

React cache() only deduplicates identical fetches within a single request and does not persist across requests. For cross-request caching of DB queries or components, wrap the function in 'use cache' with a cacheTag so Server Actions can invalidate it.